Launch the first bird high up and tap when it's lined up to hit the piece of wood. If done correctly it will take out the wooden stump and all but one pig in front (it will bounce over one). Use second bird directly to the back of the train. The train will roll forward, eventually toppling and killing the boos and the last pig on the ground. This should give you three stars.
OMG! This worked! The first bird actually took out all of the ground birds! Ty! Ty!